Sargeant will start the Saudi Arabian GP from 20th and last place after he failed to log a lap time in Q1 following a series of mishaps.
The American had a strong first run, but his time was deleted because he'd put his left front wheel over the white line as he passed the pit entry.
On his second run, he had a spectacular spin at the high-speed Turn 22, from which he was fortunate to escape without damage after stopping short of the wall.
He then made another mistake at Turn 1 at the start of his final lap and, having touched the wall with his left rear, he parked a few corners later.
Sargeant failed to set a representative time, and had to rely on the discretion of the stewards in order to be allowed to start Sunday's race.
"One small mistake by a few inches then in a space of a few minutes it spiralled into a little bit of an unfortunate session," said Robson.
"I think that's how it goes. It's very easy for that to happen. It's experience, you need to keep yourself a little bit calmer in that situation and make the following chances work for you.
"I think that is what it is to be a rookie when he's playing with such fine margins."
Sargeant admitted to some frustration after his initial time was deleted.
"I guess I just put my front left on the painted at pit entry," he said when asked by Motorsport.com about the incident.
"It's frustrating because there's been no policing of track limits all weekend, and then we have no reference coming into qualifying and I do what I've done all weekend and it gets deleted.
"But it's still my fault. I had two more laps to deliver a time, I didn't so obviously I'm disappointed in myself for that.
"The team gave me a great car, we had a lot of speed, to be honest. And yeah, it is disappointing to have not put a good lap in for them and move forward in quali."
However, he denied that the deleted time put extra pressure on him for the lap when he spun.
"Not the second lap," he said. "I was on another great lap. I was up on myself and probably just was asking for a little bit too much in Turn 22. And once I spun there, I lost the feeling on the brake pedal.
"So I obviously slammed the brakes there and then I don't know if that affected me on the next lap, but it's just it's frustrating. And I feel bad for the team because we deserved a lot more today."
Asked if he was overdriving as the need for a clean lap became more urgent, he said: "Yeah, it definitely changes. Maybe something I need to do better, it sort of changes the emotion. Just a bit frustrating.
"It's like I've just done a great lap, you take it away, and now I have to go and do it again and take it away for nothing.
"It's maybe something I can get better at. Something to look at for sure. But like I said, I didn't feel the pressure for the second lap. I was on another great lap and just made a small error."
Sargeant's Williams team-mate Alex Albon qualified 17th.
